In today's wireless communications networks a number of different technologies are used, such as Long Term Evolution (LTE), LTE-Advanced, Wideband Code Division Multiple Access (WCDMA), Global System for Mobile communications/Enhanced Data rate for GSM Evolution (GSM/EDGE), Worldwide Interoperability for Microwave Access (WiMax), or Ultra Mobile Broadband (UMB), just to mention a few possible technologies for wireless communication.
A wireless communications network typically comprises radio base stations providing radio coverage over at least one respective geographical area forming a cell. The cell definition may also incorporate frequency bands used for transmissions, which means that two different cells may cover the same geographical area but using different frequency bands. Wireless devices, also referred to herein as User Equipments, UEs, mobile stations, and/or wireless terminals, are served in the cells by the respective radio base station and are communicating with respective radio base station. The wireless devices transmit data over an air or radio interface to the radio base stations in uplink, UL, transmissions and the radio base stations transmit data over an air or radio interface to the wireless devices in downlink, DL, transmissions.
Channel-State Information, CSI, Measurement and Feedback
In many wireless communications systems, Channel-State Information, CSI, feedback is important for obtaining a good performance. In the downlink, reference signals, i.e. signals comprising reference symbols, are transmitted from a radio base station to a wireless device in order to enable the wireless device to measure the downlink channel in order to estimate the channel state. The wireless device then reports a downlink CSI feedback to the radio base station. The downlink CSI feedback typically comprise a Channel-Quality Indicator, CQI, value and a Rank Indicator, RI, value. More detailed downlink CSI reports may comprise frequency-selective CQI and/or Pre-coding Matrix Indicator, PMI, values. In the uplink, Sounding Reference Signals, SRS, are transmitted from the wireless device to the radio base station. The radio base station may then perform measurements on the SRS and thus determine the CSI information that the radio base station may, for example, use in selecting an uplink transmission format or transmission parameters, such as, e.g. pre-coding, rank, modulation and coding scheme, for uplink transmissions from the wireless device.
The 3GPP Long Term Evolution, LTE, system supports CSI-reporting schemes that rely on the Reference Symbols, RS, being transmitted periodically; Cell-specific Reference Symbols, CRS, are sent every subframe while user-specific CSI-RS may be sent with a larger periodicity. Wireless devices using Transmission Mode 10, TM10, rely solely on CSI-RS resources, while other wireless devices typically use the CRS at least for interference measurements.
Wireless devices using TM10 may be configured to report CSI for multiple CSI processes. Each CSI process may have different CSI measurement resources. A CSI measurement resource comprise a CSI Reference Symbol, CSI-RS, resource and a CSI Interference Measurement, CSI-IM, resource. Both the CSI-RS and the CSI-IM resources are divided into sets of resources, where each set is identified by a CSI-RS configuration index. Each CSI-RS configuration index comprise resources in every Physical Resource Block, PRB, in the frequency band. A subframe configuration specifies a subframe periodicity, and a subframe offset specifies for the wireless device at which time instances the respective measurement resources are available.
Massive MIMO
Future Radio Access Technologies, RATs, are expected to support a lot more transmit antennas, and then particularly on the network side in the wireless communications network. For example, in the context of “Massive MIMO”, the number of antennas is expected to be very large. This means that a single transmission point may have antennas ranging in the order of several hundreds or even thousands of antenna elements. A large, albeit much smaller, number of antennas could potentially be expected also in the wireless device at high carrier frequencies. This is because the physical size of the antenna elements at those frequencies can be made very small.
This increased number of antenna elements makes it possible to form more directive antenna patterns as compared to what is possible with current older antenna systems, Hence, wireless communications networks incorporating more capable RATs may focus its transmitted and/or received signal much more efficiently towards the wireless device being served, whilst suppressing the interference from/to other wireless devices. Each such direction is typically referred to as a ‘beam’ or “beamforming direction”, whereas the entire process may be referred to as ‘beamforming’.
Differential Beamforming
“Massive MIMO” is usually described as a technique for a Time Division Duplex, TDD, system, but not for a Frequency Division Duplex, FDD, system. This is because, for TDD, the radio channel is reciprocal and hence uplink channel estimation on the SRS additionally also provides corresponding downlink channel estimate which may be used by the radio base station to form its beam towards the wireless device. However, for FDD, obtaining a downlink channel estimate would require too much overhead. Firstly, this is because reference signals would need to be transmitted by the radio base station upon which the wireless device may perform channel measurements in order to determine its channel estimates. This alone would consume a large portion of the physical transmission resources in the wireless communications network. Secondly, the channel estimates also needs to be communicated back to the radio base station, which in turn would further consume a lot of physical transmission resources when the number of antennas is large.
An alternative solution to this problem in FDD systems may be to perform beamforming for the CSI-RS and utilize the support of multiple CSI processes. A wireless device would then be configured with multiple CSI processes, wherein the CSI-RS are beam-formed using different beams, and the CSI reported by the wireless device would then be used by the radio base station to form new beams. Hence, by utilizing the reported pre-coders and quality estimates from the wireless device, an iterative method of obtaining new beams is possible. All such iterative methods may be viewed as different forms of differential beamforming.
LTE PUSCH Power Control
In 3GPP LTE systems, power control is applied for uplink physical channels. The aim for the power control is to maintain a target received power spectral density at the receiving radio base station. For the uplink data channel, i.e. the Physical Uplink Shared Channel, PUSCH, the transmitted power by the wireless device in subframe i is determined according to the equation:PPUSCH(i)=min{PCMAX,10 log10(MPUSCH(i))+PO_PUSCH(j)+α(j)·PL+ΔTP(i)+f(i)}wherein                PCMAX is the configured maximum transmit power of the wireless device,        MPUSCH(i) is the number of resource blocks allocated for the wireless device,        PO_PUSCH(j) is a parameter consisting of the sum of a cell-specific and a user-specific part provided by higher layer,        α is cell-specific parameter configured by higher layers, also known as fractional pathloss compensation factor,        PL is the downlink pathless estimate calculated in the wireless device,        ΔTF (i) is a user-specific parameter provided by higher layers, and        f(i) is user-specific correction term controlled by TPC commands sent in uplink grants which are transmitted on the PDCCH.        
It should also be noted that for later releases of the 3GPP specifications, the power control may be slightly more complicated due to the support of multi-carrier where wireless devices may support multiple serving cells.